Rationale: The function of Nox4, a source of vascular H 2 O 2 , is unknown. Other Nox proteins were identified as mediators of endothelial dysfunction. Objective: We determined the function of Nox4 in situations of increased stress induced by ischemia or angiotensin II with global and tamoxifen-inducible Nox4 −/− mice. Methods and Results: Nox4 was highly expressed in the endothelium and contributed to H 2 O 2 formation. Nox4 −/− mice exhibited attenuated angiogenesis (femoral artery ligation) and PEG-catalase treatment in control mice had a similar effect. Tube formation in cultured Nox4 −/− lung endothelial cells (LECs) was attenuated and restored by low concentrations of H 2 O 2, whereas PEG-catalase attenuated tube formation in control LECs. Angiotensin II infusion was used as a model of oxidative stress. Compared to wild-type, aortas from inducible Nox4-deficient animals had development of increased inflammation, media hypertrophy, and endothelial dysfunction. Mechanistically, loss of Nox4 resulted in reduction of endothelial nitric oxide synthase expression, nitric oxide production, and heme oxygenase-1 (HO-1) expression, which was associated with apoptosis and inflammatory activation. HO-1 expression is controlled by Nrf-2. Accordingly, Nox4-deficient LECs exhibited reduced Nrf-2 protein level and deletion of Nox4 reduced Nrf-2 reporter gene activity. In vivo treatment with hemin, an inducer of HO-1, blocked the vascular hypertrophy induced by Nox4 deletion in the angiotensin II infusion model and carbon monoxide, the product of HO-1, blocked the Nox4-deletion-induced apoptosis in LECs. Conclusion: Endogenous Nox4 protects the vasculature during ischemic or inflammatory stress. Different from Nox1 and Nox2, this particular NADPH oxidase therefore may have a protective vascular function.